In the conventional practice of photography, a light sensitive material, i.e. silver halide grain, is exposed to actinic radiation to form what is referred to in the art as a "latent image". The "latent image" is the invisible precursor of a useful visible image that appears upon photographic development; it is a record of the exposure that is invisible to the unaided eye.
Once the light sensitive material has been exposed, it is then developed to produce a visible image. Development occurs by subjecting the light sensitive material to a reducing agent (developer). The reducing agent is oxidized and, in the course of being oxidized, causes an exposure-dependent fraction of the silver halide to be converted to metallic silver. This metallic silver is visible and, in black and white photography, it forms the visible record. In color photography, the oxidized reducing agent reacts with a dye precursor (coupler) to form a visible dye record.
Because formation of the visible image is exposure dependent, it is a primary object in the photographic field to enhance the light sensitive material's sensitivity to light. Increased light sensitivity results in an increased density of the visible image for each given level of exposure. It allows for images to be photographically recorded in poorly lit environments. Moreover, it allows for the construction of films yielding superior image quality.
It is known to improve the sensitivity (also referred to as "speed") of silver halide grains--which are typically contained in a gelatinous emulsion--by incorporating into their internal structure various non-silver or non-halide impurities. Such impurities are termed dopants, and they are typically added during silver halide precipitation. Examples of dopants can be found in European Patent Applications 0325235 and 0475298, and in U.S. Pat. Nos. 4,835,093 and 4,933,272.
It is also known to incorporate impurities onto the surfaces of silver halide grains in order to achieve sensitization. Such impurities are termed grain surface modifiers; they are added to the photographic emulsion after precipitation or ripening; and they are exemplified in U.S. Pat. Nos. 5,256,530 and 5,252,451.
Other means of increasing the sensitivity of silver halide emulsions include adjusting the pH and/or the pAg of the emulsion. Also, enhanced photographic sensitivity can be obtained by the addition of certain types of chemical sensitizers, typically as grain surface modifiers. Several types of sensitizers have been identified. The most prevalent are gold and sulfur compounds, both of which are thought to enhance emulsion speed by forming electron or photohole traps on the silver halide crystal surface.
Sensitization has also been accomplished by the addition of other transition metals. Specifically, platinum salts have been used, although sensitization with such salts is strongly retarded by gelatin. In addition, iridium salts and complex ions of rhodium, osmium, and ruthenium have been used as chemical sensitizers. The overall effect of these metals on sensitivity appears to be dependent upon their valence state.
Reduction sensitization is another means by which sensitivity enhancement can be obtained. Known reducing agents include stannous chloride, ascorbic acid (as in European Patent Applications 0369491 and 0369424) and dimethylamine borane (U.S. Pat. Nos. 4,150,093 and 3,782,959).
The use of sensitizers as described above often causes an indiscriminate, and undesirable, reduction of silver ions to silver atoms. The silver atoms, in turn, give rise during development to a generally or locally developed density that is not associated with the action of the image forming exposure. This density is typically termed fog and, for negative elements, it is best measured as the minimum density on the element's D-Log E curve (D.sub.min); for positive (reversal) element's, it is best measured as the percent of Dmin relative to Dmax in a standard 6 minute E6 rehalogenation process.
In sensitized photographic silver halide elements, fog formation is often controlled by the addition of oxidants which impede the reduction of silver ions to metallic silver (or reverse the reaction). Addition of such oxidants, however, requires that a balance be struck between the desensitization effects of the oxidants and the sensitization effects of the sensitizer, a balance which is difficult to attain.
It is therefore desired to provide photographic light sensitive elements which exhibit increased sensitivity, and which do so without requiring the addition of oxidants to control fog formation.
Photographic silver halide elements, particularly sensitized ones, often also suffer from desensitization when subjected to mechanical pressure. This problem is severe in elements with silver halide grains exceeding 1 micron in size; and it is most acute in elements with silver halide grains having a tabular morphology.
It is therefore also desired to provide photographic light sensitive elements which exhibit increased sensitivity, but which do so without further exhibiting substantial desensitization when subjected to mechanical pressure.